The term “rapid prototyping” (RP) covers generative manufacturing processes in which 3-dimensional models or components are prepared from computer-aided design data (CAD data) (A. Gebhardt, Vision of Rapid. Prototyping, Ber. DKG 83 (2006) 7-12). These are processes such as e.g. stereolithography (SL), selective laser sintering (SLS), 3D printing, fused deposition modelling (FDM), ink-jet printing (IJP), 3D plotting, multi-jet modelling (MJM), solid freeform fabrication (SFF), laminated object manufacturing (LOM), laser powder forming (LPF) and direct ceramic jet printing (DCJP), with which models, components or shaped parts can be prepared cost-effectively even on a small scale (A. Gebhardt, Generative Fertigungsverfahren, 3rd edition, Carl Hanser Verlag, Munich 2007, 77 et seqq.). Stereolithography involves RP processes (A. Beil, Fertigung von Mikro-Bauteilen mittels Stereolithographie, Düsseldorf 2002, VDI-Verlag 3 et segq.) in which a shaped part is constructed in layers from a liquid and curable monomer resin on the basis of CAD data.
Stereolithographic processes for the preparation of dental shaped bodies such as inlays, crowns or bridges are advantageous above all in the case of ceramic materials, because the impression-taking and casting processes, which involve considerable manual outlay in the dental laboratory, and the milling and grinding operations can thus be greatly simplified and at the same time the large loss of material which occurs with non-generative processes can be avoided. As a complete digital process chain is in place today, the standard process steps for the preparation of e.g. multi-unit bridge frameworks (alignment in the articulator, wax modulation, embedding and casting) can be replaced by the digitalization of the model, virtual design of the dental shaped body and generative stereolithographic manufacture thereof.
In the stereolithographic preparation of ceramic shaped parts a ceramic green compact is firstly prepared by layered radiation curing of a free-flowing ceramic slip which is then sintered after the debinding to form a dense ceramic shaped body. The green compact is also called a green body. The elimination of the binder is called debinding. Here, the binder used is usually removed by heating the green compact to a temperature of approx. 80° C. to 600° C. It is essential that the formation of cracks and deformations is avoided as far as possible. The green compact becomes the so-called white body as a result of the debinding. During the debinding, the binder is broken down into volatile components by means of thermal and thermo-chemical processes.
The debinding is a critical step in the process. Here, the danger is high that the component will be damaged by gases which form in the decomposition of the organic matrix and by the pressure exerted by said gases. The danger is particularly high that small defects between the individual construction layers lead, during the debinding, to cracks or even to the complete destruction of the component. This risk can be reduced by increasing the debinding period, which extends the process time greatly, however.
The sintering of the white body takes place in the sintering furnace during high-temperature firing. Thereby, the finely dispersed ceramic powder is compacted and solidified by exposure to a temperature below the melting temperature of the main components, as a result of which the porous component becomes smaller and its strength increases.
U.S. Pat. No. 5,496,682 discloses light-curable compositions for the preparation of three-dimensional bodies by stereolithography, which comprise 40 to 70 vol.-% ceramic or metal particles, 10 to 35 wt.-% monomer, 1 to 10 wt.-% photoinitiator, 1 to 10 wt.-% dispersant and preferably also solvent, plasticizer and coupling agent.
U.S. Pat. No. 6,117,612 describes resins for the stereolithographic preparation of sintered ceramic or metal parts. The resins have a viscosity of less than 3000 mPa·s. For their preparation, monomers with low viscosity are used, preferably in aqueous solution. A high solids content with low viscosity is to be achieved through the use of dispersing agents.
DE 10 2005 058 116 A1 discloses suspensions for the stereolithographic preparation of ceramic implants in the manner described in U.S. Pat. No. 6,117,612 which do not contain diluting agents such as water or organic solvents as the latter are to increase the viscosity through local evaporation when energy is introduced. The viscosity of the suspension is adjusted at less than 20 Pa·s by varying the concentration of a dispersant. Alkyl ammonium salts of copolymers with acid groups are used as dispersants, wherein said dispersants can also be coated onto the particles of the ceramic powder.
Methods and compositions for the stereolithographic preparation of ceramic components are described in US 2005/0090575 A1. It is stated that shaped parts prepared with the liquid materials known from U.S. Pat. No. 5,496,682 are soft and therefore require an additional curing step in order to avoid deformations during firing, while shaped bodies obtained from paste-like materials develop internal stresses during debinding which lead to cracks during sintering. To avoid these problems, plasticizers are used and the quantity of the ceramic powder is chosen such that the viscosity of the compositions is at least 10,000 Pa-s.
From EP 2 233 449 A1, slips for the preparation of ceramic shaped parts by hot-melt inkjet printing processes are known which comprise ceramic particles, wax and at least one radically polymerizable wax and which yield green bodies which can be debound without the formation of cracks. A disadvantage of these slips is that, in the liquid state, they tend to separate on standing for a long time. In the case of hot-melt inkjet processes this is not critical because the slips are only present in liquid form during the printing process, i.e. for a relatively short period of time. However, in the case of stereolithographic processes, the slips must be stable in liquid form over longer periods of time, i.e. in particular, the particles dispersed in the slip must not settle prematurely, which represents a particular problem in respect of the highest possible proportion by volume of ceramic particles in the slip which is sought.
Furthermore, slips for stereolithographic processes are to have a high reactivity and thus enable short exposure times and process times. They must ensure a good green body strength and good dimensional stability, high accuracy and precision after debinding, sintering and final cleaning of the bodies.